1. Field of the invention
The present invention relates to improved closed cycle refrigeration systems, particularly for removing excessive hydrocarbons or other chemicals from air-vapor mixtures by condensation.
2. Related information
Environmental concerns have led to a requirement for the reduction of chemical vapors, particularly hydrocarbons emitted into the atmosphere from every possible source. One of the sources targeted are bulk loading and unloading terminals. In these terminals a vapor recovery system is used to gather the air containing hydrocarbons or other vapors which are emitted during loading and unloading operations. The vapors are commonly separated from the air by refrigeration of the air-vapor mixture to cryogenic temperatures. The refrigeration systems can reduce "condensable" hydrocarbons or other condensable materials in the vented air to acceptable levels, i.e., less than 80 mg/l.
The cryogenic temperatures required, i.e., about -100.degree. F., necessitate efficient refrigeration to achieve the separation. Since the total amount of refrigeration, expressed in tons (1 ton=12,000 BTU removed), is small when compared to large scale plants, such as ice plants and gas liquefaction plants, less efficient closed refrigeration cycles can be used with the common fluorocarbon refrigerants such as Freon 12.
With less efficient refrigerants, greater circulation and more compression power is required. Cascade refrigeration, using two refrigerants--one to cool the other--is an alternative. The most commonly used closed refrigeration cycle in these smaller scale plants is the compression of the refrigerant followed by cooling at the higher pressure and expansion across a Joules-Thomson expansion valve, the cycle being used to cool both refrigerants in the cascade refrigeration system. However, with the advent of concerns about damage to the atmosphere attributed to the fluorocarbons, these are being banned or expected to be banned in commercial operations.
A turboexpander may be used when the refrigerant is at a "free" higher pressure. As disclosed in commonly owned co-pending application Ser. No. 07/354,860 filed May 22, 1989, now U.S. Pat No. 4,923,492, the disclosure of which is fully incorporated by reference herein. Included in that disclosure are two desiccant type dryers operated in parallel to remove water from the vapors which would otherwise freeze at the cryogenic temperatures utilized. The desiccant type dryers must be periodically regenerated because their capacity is limited by the volume and adsorbing capacity of the desiccant used.